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While slow waves and sleep spindles are present in stages 2 and 3, stage 2 sleep is characterized by a higher prevalence of spindles, while slow waves dominate the EEG during stage 3. [22] [21] Slow-wave sleep is an active phenomenon probably brought about by the activation of serotonergic neurons of the raphe system. [23]
The optimal nap duration is around 10–20 minutes, as researchers have proven that it takes at least 30 minutes to enter slow-wave sleep, the deepest period of sleep. [46] Napping too long and entering the slow wave cycles can make it difficult to awake from the nap and leave one feeling unrested. This period of drowsiness is called sleep inertia.
Polysomnogram demonstrating slow-wave sleep. High amplitude EEG is highlighted in red. Slow-wave sleep (SWS), also known as Stage 3, is characterized by a lack of movement and difficulty of arousal. Slow-wave sleep occurring in both hemispheres is referred to as bihemispheric slow-wave sleep (BSWS) and is common among most animals.
NREM parasomnias are arousal disorders that occur during stage 3 (or 4 by the R&K standardization) of NREM sleep—also known as slow wave sleep (SWS). They are caused by a physiological activation in which the patient's brain exits from SWS and is caught in between a sleeping and waking state.
Sleep stages are characterized by spectral content of EEG: for instance, stage N1 refers to the transition of the brain from alpha waves (common in the awake state) to theta waves, whereas stage N3 (deep or slow-wave sleep) is characterized by the presence of delta waves. [107] The normal order of sleep stages is N1 → N2 → N3 → N2 → REM.
And slow-wave sleep, or stage 3 of non-REM sleep, is especially vital for restorative rest. Most people get around 1.5 to two hours of slow-wave sleep if they get eight hours of shut-eye.
The cause of sleepwalking is unknown. A number of, as yet unproven, hypotheses are suggested for why it might occur, including: delay in the maturity of the central nervous system, [6] increased slow wave sleep, [28] sleep deprivation, fever, and excessive tiredness. There may be a genetic component to sleepwalking.
An indicator of sleep propensity can also be seen in the shortening of the transition from light stages of non-REM sleep to deeper slow-wave oscillations. [69] On average, the latency in healthy adults decreases by a few minutes after a night without sleep, and the latency from sleep onset to slow-wave sleep is halved. [69]